The present invention relates to a rib and in particular, but not exclusively, a rib for a wing of a large aircraft, and to methods for the manufacture thereof.
Ribs typically form part of a structure that supports and defines the shape of an aerofoil surface. For example, ribs are may be provided in the aircraft wing or wing box of an aircraft, the ribs providing support for the upper and lower wing skins. A rib of the prior art will now be described. The rib comprises a generally planar web structure disposed between a series of rib feet associated with the upper wing skin and a series of rib feet associated with the lower wing skin. The rib feet are, in use, attached to other components of the aircraft wing box, such as stringers. The rib feet at one end of the rib project at an angle of less than 90 degrees from the surface of the main rib body, with the rib feet associated with the upper wing skin extending from the same side of the web as the rib feet associated with the lower wing skin. The rib is machined from a solid cuboidal one-piece block of metal material, known as a billet, in order to provide strength and to remove the problems associated with joining components that are made separately. The rib web is formed from metal that lies near the surface on a first side of the billet, and the ends of the rib feet are formed from metal near the surface on the opposite side. Thus a relatively small amount of machining is conducted on the first side of the billet to form the web. Such a rib may suffer from distortion problems which have to be accounted for during the rib machining process because the metal at and close to the surfaces of the billet is subject to residual stresses (resulting from the process used to manufacture the billet) that may cause deformation of, or undesirable internal stresses in, the web produced therefrom.
Furthermore, such ribs are usually provided with stiffeners that add strength to the rib. In the above-described rib of the prior art, these stiffeners project from one side of the web and are formed from material in the billet on the opposite side of the web to the first side of the billet. Thus, whilst the inclusion of such stiffeners does not require a larger billet, the neutral axis of the stiffeners is displaced from the plane of the web. As a result, the stiffeners need to be stronger, and therefore larger and heavier, than stiffeners provided on either side of the web.
One possible way to reduce such problems is to provide an extra (second) series of rib feet associated with the upper wing skin and an extra (second) series of rib feet associated with the lower wing skin, these extra rib feet projecting from the opposing side of the rib web from which the first series of feet project. Such a rib has a substantially ‘H’ shaped cross-section and suffers less from some of the problems of the afore-mentioned rib of the prior art, but suffers from the problem that two sets of rib feet are associated with each wing skin thus increasing the complexity of the machining task, increasing the weight of the rib, and requiring a larger and more expensive billet from which to machine the rib. As the size of the billet (especially in terms of its thickness) increases, the structural properties of the resulting metal may not be as good as those of a billet of a smaller size. Thus, if the billet has to be larger to accommodate a certain shape and size of rib, then the rib may need to be made more massive in order to compensate for the reduced structural strength of the metal from the larger-sized billet.